This invention relates to a stable zinc electrode for use in a secondary battery having an alkaline electrolyte. More particularly, this invention relates to a zinc electrode comprising a particular porous material for entrapping zinc species in the discharged state and retaining it near the electrode for recovery during recharging.
During discharge of a zinc/nickel oxide battery, zinc of the negative electrode forms zincate anion, Zn(OH).sub.4.sup.-2, that dissolves in the alkaline electrolyte and diffuses throughout the cell. When the cell is recharged, the zinc is mostly recovered, but its distribution on the electrode is significantly different. Active zinc material is lost from the perimeter of the electrode and redeposited towards the center. This shape change reduces effective surface area of the electrode and thereby cell capacity. The redeposited zinc is denser and less porous, which causes the electrode to passivate at low current densities. Also, zinc dendrites may form and penetrate the separator to create short circuits within the cell. Thus, zincate ion diffusion substantially reduces the useful life of the cell.
Calcium hydroxide has been added to zinc electrodes to minimize the solubility of the zincate anion and thereby stabilize the electrode. However, the required amount of calcium hydroxide significantly thickens the electrode and lowers the energy density of the cell. Also, calcium hydroxide is only effective for relatively low electrolyte alkaline concentrations, for example, about 20 weight percent. Higher alkaline concentrations of 30% or greater are desired to reduce electrolyte resistance and freezing point.
Therefore, it is an object of this invention to provide a shape-stable zinc electrode for extending the useful life of an electrochemical cell wherein the zinc electrode is in contact with an alkaline electrolyte. The electrode traps zincate ions during discharge and retains the zinc near its predischarge location for recovery during recharging. Thus, zinc at the electrode perimeter prior to discharge is recovered near the perimeter, and does not diffuse into the bulk of the electrolyte. A relatively small amount of the zinc-trapping material improves the distribution and density of the redeposited zinc, without significantly interfering with cell performance.
It is also an object of this invention to provide an improved zinc electrode for use in a cell having a relatively concentrated alkaline electrolyte, for example, 30% or more potassium hydroxide, which electrode comprises a highly porous insoluble silicate material evenly distributed throughout the electrode to act as a reservoir for storing zincate ions near where they are formed. The material physically traps zincate ions, in contrast to electrode additives which seek to tie up zincate ions chemically.